Some key elements of the SAE/EPC network have been dealt with in the following references:    [1] TS 23.236: Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes;    [2] TS 23.401: General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access;    [3] TS 24.301: Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3;    [4] TS 23.007: Restoration Procedure;    [5] TS 29.274: 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3    [6] TR 23.857.
With the migration towards all IP (Internet Protocol) networks, all mobile traffic (voice and data) will be carried on the packet core network step by step. In FIG. 1, the procedure in an EPC network for Mobile Terminating (MT) voice service, when the UE is in Idle state is shown according to 3GPP TS 23.401, chapter 5.3.4.3. The mobile terminated traffic can be voice, data or multimedia traffic. In this procedure, the following events may be undertaken:
(101) The User Entity, UE, attaches and registers in the SAE/EPC network.
(105) The PDN (Packet Data Network) Gateway, P-GW, receives a data packet addressed to the UE (IP address) and forwards the data packet to the Serving Gateway, S-GW, via the corresponding GTP (GPRS Tunnelling Protocol) tunnel of the UE.
(108) S-GW sends Downlink Data Notification to Mobile Mobility Entity, MME.
(110) MME replies to S-GW with Downlink Data Notification Ack (Acknowledge).
(112) UE location is known by MME with an accuracy of the serving Tracking Area. MME pages UE in the Tracking Area.
(114) When eNode B, eNB, receives paging messages from the MME, the eNB initiates Paging procedure over the air.
(116) Upon reception of paging indication, the UE initiates UE triggered Service Request procedure (specified in TS 23.401) as the paging response.
(120) The downlink data is sent to the UE.
MT Call Problem in Case S-GW of Restart
According to the current mechanism described in 3GPP TS 23.007, when a P-GW/MME/SGSN detects that a peer S-GW has restarted, it shall delete all PDN (Packet data Network) connection table data/MM bearer contexts associated with the peer node that fail as well as freeing any internal PGW/MME/SGSN resources associated with those PDN connections. Since there are no bearer/MM (Mobility Management) contexts in the PGW, the mobile terminated service can not be delivered to the UE for a long time.
TR 23.857 proposes the following alternative solution to recover the bearer and MM contexts after detection of a SGW failure:
1) The MME/S4-SGSN (Serving GPRS Support Node) and PGW maintain the bearers and MM contexts after detection of SGW failure. If the MME/S4-SGSN detects the SGW failure or restarts by referring to restart counter, instead of removing associated resources, the MME/S4-SGSN and PGW maintain the bearers and MM contexts. The MME/S4-SGSN needs to know whether the PGW supports the capability to maintain the old PDN connection after SGW failure or restarts, vice versa: which can be learned based on local configuration or new signalling between MME and PGW. This makes it possible for the MME/S4-SGSN to select a new SGW for the UE.2) The MME/S4-SGSN selects a new SGW for the ECM_IDLE (EPS Connection Management—IDLE) UE.
The MME/S4-SGSN selects a new SGW for the ECM_IDLE UE based on the last visited TAI/RAI (Tracking Area Identity/Routing Area Identity). Then MME/S4-SGSN sends a Create Session Request message to the new SGW to establish the bearer contexts. The new SGW will send a Modify Bearer Request message to the PGW to update the bearer context in the PGW.
3) The MME/S4-SGSN releases S1/Iu resources and selects a new SGW for the ECM_CONNECTED UE.
The MME/S4-SGSN firstly releases S1/Iu resources of ECM_CONNECTED UE. Then the MME/S4-SGSN will handle the UE in the same way as for the ECM_IDLE UE.
However, considering typical load conditions, the inventors note that the above recovery can not be finished for all the UEs immediately. In case there is an incoming MT (mobile terminated) call arriving at PGW before the recovery occurs, the MT call will fail according to the above described mechanism.